The human spine is a complex structure designed to achieve a myriad of tasks, many of them of a complex kinematic nature. The spinal vertebrae allow the spine to flex in three axes of movement relative to the portion of the spine in motion. These axes include the horizontal (i.e., bending either forward/anterior or aft/posterior), roll (i.e., lateral bending to either the left or right side) and rotation (i.e., twisting of the shoulders relative to the pelvis).
The intervertebral spacing (i.e., between neighboring vertebra) in a healthy spine is maintained by a compressible and somewhat elastic disc. The disc serves to allow the spine to move about the various axes of rotation and through the various arcs and movements required for normal mobility. The elasticity of the disc maintains spacing between the vertebrae, allowing room or clearance for compression of neighboring vertebrae, during flexion and lateral bending of the spine. In addition, the disc allows relative rotation about the vertical axis of neighboring vertebrae, allowing twisting of the shoulders relative to the hips and pelvis. Clearance between neighboring vertebrae, typically maintained by a healthy disc, is also important to allow nerves from the spinal cord to extend out of the spine between neighboring vertebrae without being squeezed or impinged by the vertebrae.
In situations (e.g., based upon injury or otherwise) where a disc is not functioning properly, the inter-vertebral disc tends to compress, and in doing so pressure is exerted on nerves extending from the spinal cord by this reduced inter-vertebral spacing. Various other types of nerve problems may be experienced in the spine, such as exiting nerve root compression in neural foramen, passing nerve root compression, and enervated annulus (where nerves grow into a cracked/compromised annulus, causing pain every time the disc/annulus is compressed), as examples.
Many medical procedures have been devised to alleviate such nerve compression and the pain that results from nerve pressure. Many of these procedures revolve around methods to prevent the vertebrae from moving too close to each other by surgically removing an improperly functioning disc and replacing the improperly functioning disc with a lumbar interbody fusion device or spacer. Although prior interbody devices, including spacers, can be effective at improving patient condition, a significant amount of time is required for recovery from the surgery and for ultimately fusing the adjacent vertebrae together with the aid of the device. Damage or deterioration of the disc(s) may also cause distortion of the normal curvature of the spine. Indeed, it is known that the lordosis and kyphosis of the spine can be affected by irregular discs.
Generally speaking, reducing the time that is required for the patient to recover from the operation and for the vertebra to fuse together will reduce patient discomfort and hasten the day that the patient can resume normal activities. What is needed, therefore, are spacers that promote recovery and bone fusion.